> It's not unprecedented to have an open source license with revocation or termination clauses, either. I recall seeing ones that basically say "If you file a patent suit around this open software, your rights to use it are gone".
Trying to take back the license based on use of the software, however, would make it not "open source", since that would be use restriction.
Mozilla has had their own dedicated license - the Mozilla Public License - for as long as I remember. My understanding is that FF and Thunderbird's source code are both still under this license.
Whether or not the MPL counts as 'open source' is a question for the people who steward that term, I guess. But they've not been using a Standard Open Source License for a while.
Trying to take back the license based on use of the software, however, would make it not "open source", since that would be use restriction.